The present invention concerns an air knife designed to be used in coating machines or equivalent to finish and smooth the coating by blowing air onto it, e.g. to remove excess coating material, said air knife comprising a main channel into which the air is blown in the axial direction of the channel at least from one direction, the main channel being provided with an air slit of a length essentially equal to the width of the web and directed towards the web to be coated, the air being blasted through said slit onto the web preferably via a nozzle part provided with a nozzle slit, said main channel being provided with a partition dividing the channel into two parts.
An air knife is a device used e.g. in a coating machine to remove the excess coating material from a web of paper or cardboard by means of a sharp jet of air blown at the web surface at a high speed. The air knife is generally placed in the coating or finishing machine in such a way that the air jet meets the web at a certain angle against its running direction, the web being supported from its dry side by a roller placed after the coating station. In this way the impulse of the air jet is transferred to the coating material to be removed and the velocity of the excess material in front of the jet is reduced from the web speed to zero, so that the excess material will then drip off from the web surface.
In addition to removing the extra coating material, it is obvious that the characteristics of the air flow, e.g. turbulence, have an influence on the quality of the remaining surface of the web. For the air knife to have an equal effect over the whole width of the web, the air flow obviously has to be evenly distributed over the whole web width and that the air velocity should have no transverse component relative to the direction of the web.
These requirements are not met adequately by the currently used constructions. The main channel of the air knife, usually extending over the whole width of the web, is of a constant sectional form and the air is supplied into the channel from one or both of its ends. In fact, air supply from both ends means that two nozzle channels are used, each extending over half the width of the web.
Such a construction cannot provide the above-mentioned properties of an ideal air knife. If the main channel has a constant section, the velocity of the air decreases in the channel in the direction of flow of the air and reaches zero at the opposite end of the tube (or at the middle if air is blown in from both ends). Therefore, the static pressure of the air increases in the direction of air flow and, consequently, the air flow is unevenly distributed in such manner that the flow rate is at its lowest at that end where the air enters the channel and at its highest at the opposite end (or, if air is blown in from both ends) at the middle. Moreover, the direction of flow of the air is not exactly the desired direction. Since the impulse acting in the direction of the main channel cannot be lost, the outcoming air flow also has a velocity component transverse to the web direction and equal to the local velocity of the air in the main channel. Thus, the difference between the actual direction and the desired direction of the air flow is largest at that end of the nozzle slit which lies closer to the point of supply of air.